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Build performance

When we talk about build performance, it is important to understand that there are several build phases:

  • cold build (booting your app up for the first time)
  • warm build (booting your app up when cache was populated)
  • rebuild (subsequent rebuilds that happen on file change)

Cold build is the slowest because the cache is not yet populated and the application is booting for the first time. Build time varies and depends on the number of dependencies that application has but ballpark should be around 5 seconds for small to middle size applications and around 15 seconds for large size applications.

Warm build is faster then cold one because the cache was populated already and it takes less time re-compute dependencies. Build time varies but ballpark should be around 2 seconds for small to middle size applications and around 10 seconds for large size applications.

Rebuild aims to be the fastest because it happens the most often. App/ JavaScript Rebuild time varies but ballpark should be around 200-300ms for small to middle size applications and up to 1 second for large size applications (200kloc js + 3000 modules).

  • rebuild of JS in app/ <--- Largely our focus, as this is likely the most common.
  • rebuild of CSS/Sass/Less in app/: largely depends on 3 factors
    • the size of bower_components (we have plans to largely mitigate this one)
    • which preprocessor is being used (libsass vs ruby-sass vs less vs ...)
    • the amount of css
  • rebuild of vendor/, bower_components/ <--- somewhat costly still, due to how slow available sourceMap Libraries are. We have WIP with a more v8/JIT friendly sourceMap lib, already showing some very nice improvements.

note these times are based on a posix box with and SSD, win32 unfortunately tends to be slower. As we continue to improve performance, both posix and win32 improve, hopefully future work will bring these platforms build times closer together.

If you see that application timings escape, there might be a problem.

Addons known to cause a slow down (but have not yet been addressed):

  • any old non-patch based broccoli plugin
  • ember-cli-component-css
  • ember-browserify
  • ... ?

Make sure to mention which type of the build seems to be the problem so we can help identify and fix issues faster.

FAQ/Common Issues & Solutions:


My rebuilds are slow. (And I have anti-virus installed)


Our build-system assumes a relative fast/performant file system (although, we continue to reduce IO related work). It is quite common for a Anti-virus to slow down IO.

Common issues:

  • anti-virus scanning of /tmp/, oftentimes this can be avoided altogether.
  • anti-virus on-file-access re-scanning files, oftentimes this can be disabled for the app directory. or ember can be whitelisted.

My rebuilds are slow. (And I am using an encrypted thumb-drive to host my project)


These sorts of drives are notoriously slow. Although we continue to reduce our IO overhead, you will be running at a disadvantage. Oftentimes, a much better alternative is hardware supported full-disk encryption, like on most OSX corporate laptops (mine included) use. This setup is both reasonably secure, and has negligible impact on performance.


My JavaScript rebuilds are still slow.


please run

npm ls broccoli-funnel broccoli-merge-trees broccoli-filter broccoli-persistent-filter broccoli-concat broccoli-caching-writer

and ideally the following should be true (otherwise some upgrades may be required)

  • broccoli-funnel should be at ^1.0.1
  • broccoli-merge-trees should be at ^1.1.0
  • broccoli-persistent-filter should be at ^1.1.6
  • broccoli-filter often needs to be replaced with broccoli-persistent-filter (we hope to re-merge the two eventually)
  • broccoli-sourcemap-concat should be at ^2.0.2 but will soon be replace by broccoli-concat (we have just re-merged the two)
  • broccoli-caching-writer should be at ^2.2.1
  • broccoli-concat should be at ^2.0.3
  • broccoli-stew should be at ^1.2.0
  • likely more...

Up next we should check for old and deprecate plugins

  • npm ls broccoli-static-compiler this should no longer be used, rather broccoli-funnel at v1.0.1 should be used
  • ..

npm v3 made my build slow


Well what happened is npm v3 changed the module topology, this coupled with a misbehaving plugin may result in extra files (maybe all of node_modules) being pulled into the build. This is going to be slow.., the solution is to find the offending plugin, and upgrade (or report the issue if it is not yet fixed).

One such plugin is ember-cli-ic-ajax, which has been fixed. So please be sure to upgrade.

Finding such plugins, we can use a series of DEBUG flags, to gain more insight

DEBUG=broccoli-funnel:Funnel*Addon* ember s should reveal if extra files are being pulled into the build


My builds are slow, and the above Q/A hasn't helped


Please be sure to read this full document (including the tips and tricks bellow). If the issue persists, please report an issue.

Be sure to include:

  • npm version
  • npm ls (as a gist)
  • ideally a reproduction
    • we are aware some are unable to share apps (even privately), this may prove more difficult to debug. Although in some cases, consulting and proper IP related paperwork to allow sharing could enable improved debugging

My builds are slow for a reason not mentioned here


We would love a PR improving this guide.

Various tips and tricks

How to explore/debug and hopefully address performance issues

DEBUG logging

We use heimdalljs-logger for logging, which supports the same usage as the de facto standard debug. Quite often this can be used to quickly discover obviously wrong things.


  • DEBUG=<pattern> ember s
  • DEBUG=* ember s for all logging (this will be very verbose)
  • DEBUG=ember-cli* ember s for all ember-cli logging
  • DEBUG=broccoli* ember s for all broccoli logging
  • DEBUG=broccoli*,ember-cli* ember s for both broccoli and ember-cli logging

The above patterns will be very verbose. But to make them even more verbose you can set the log level via DEBUG_LEVEL

  • DEBUG=* DEBUG_LEVEL=debug ember s

To make them a bit less verbose, a curated set of performance related logging flags are:

  • DEBUG=broccoli-caching-writer:* ember s
  • DEBUG=broccoli-funnel:* ember s
  • DEBUG=broccoli-funnel:Funnel*Addon* ember s
  • DEBUG=broccoli-merge-trees ember s
  • DEBUG=broccoli-merge-trees:TreeMerger* ember s
  • DEBUG=broccoli-merge-trees:Addon* ember s
  • DEBUG=broccoli-merge-trees:styles ember s
  • DEBUG=broccoli-merge-trees:compileTemplates* ember s
  • DEBUG=broccoli-merge-trees:compileTemplates* ember s

Because many plugins are used repeatedly it may be difficult to see the context for log entries. By default, 3 nodes of context are shown.

DEBUG_LEVEL=debug DEBUG=broccoli-merge-trees: ember build
broccoli-merge-trees: [TreeMerger (testFiles)#777 -> ConcatWithMaps#782 -> BroccoliMergeTrees#783] deriving patches

To show more (or fewer) lines of context, specify the environment variable DEBUG_DEPTH.

DEBUG_DEPTH=5 DEBUG_LEVEL=debug DEBUG=broccoli-merge-trees: ember build
# => broccoli-merge-trees: [TreeMerger (allTrees)#1 -> BroccoliMergeTrees#668 -> TreeMerger (testFiles)#777 -> ConcatWithMaps#782 -> BroccoliMergeTrees#783]

[... ConcatWithMaps#782 -> BroccoliMergeTrees#783] means that the log entry occurred in broccoli merge-trees node with id 783, whose parent was a concat with maps node with id 782. These ids are shown in the visualization graph. See Visualization for details.

... more on what to look for ...



To visualize build tree, we use graphviz. To install it run brew install graphviz or download it directly from here.

You will also need to install broccoli-viz version 4.0.0 or higher. npm install -g broccoli-viz@^4.0.0.

To generate visualization:

  • BROCCOLI_VIZ=1 ember build
  • broccoli-viz >
  • dot -Tpng >

Each build will generate an additional graph,<build-number>.json

in-depth look

in-depth tooling, aimed to provide much deeper insight into the given build

  • dot: is the input to graphviz, allowing tree visualization
  • json: more detailed counts and timings related to the corresponding build